These make use of the cas*, ld* and swp instructions added in ARMv8.1.
Testing shows them to be significantly more performant than LL/SC-based
implementations.
No functional change here since the wrappers still unconditionally
select the _llsc variants.
Reviewed by: andrew, kib
MFC after: 1 month
Submitted by: Ali Saidi <alisaidi@amazon.com> (original version)
Differential Revision: https://reviews.freebsd.org/D23324